Mono-, di- and trialkyl derivatives of "sulfabenzamide" (N-4-aminophenylsulfonyl-benzamide) are prepared and their EI mass spectra are examined. It is found that the fragmentation of N- alkylsulfabenzamides (Alkyl = CH3 to n-C5H11) proceeds via a very specific rearrangement process involving migration of carbonyl O-atom to sulfur. The proposed mechanism involves an intermediate formation of distonic molecular ions, and the driving force for this process is the formation of stable N-alkylcyanobenzene cations [R-N+≡CC6H5]. The findings are confirmed by exact mass measurement, MS/MS experiments and deuterium labeling.